Optical filters that sufficiently transmit light in the visible wavelength region (hereinafter, referred to as visible light) while blocking light in the near-infrared wavelength region (hereinafter, referred to as near-infrared light) have recently been used in various applications.
For example, a solid state image sensing device (CCD, CMOS, or the like) is used in an imaging device such as a digital still camera. Further, an optical filter is disposed between an imaging lens and the solid state image sensing device so that the sensitivity of the solid state image sensing device becomes similar to the human visibility.
As an optical filter for imaging devices, it is known a glass filter including near-infrared absorbing glass that selectively absorbs near-infrared light and in which CuO or the like is added to fluorophosphate-based glass or phosphate-based glass. Further, a film made of a transparent resin film containing a dye that absorbs near-infrared light and an optical filter including this film stacked on a glass substrate have been developed.
In order to more accurately block near-infrared light, some of these optical filters include, in addition to the above-described structure, a multilayer film that reflects near-infrared light, in particular, 700 to 1200 nm wavelength light, for instance. It is known, however, that a spectral transmittance curve of the reflective multilayer film shifts because its transmittance varies depending on an incident angle.
Under such circumstances, Patent Reference 1 (International Publication WO2015/054864) discloses an optical filter that achieves a reflectance of 70% or more for 45° incident light in an 800 to 1200 nm wavelength region. That is, in the optical filter of Patent Reference 1, a high reflectance for near-infrared light whose incident angle ranges from 0° to 45° is obtained in a wide band.